All but the simplest electronic or opto-electronic components require some lateral definition of one or more semiconductor or insulating layers. Similarly, electronic or optoelectronic integrated circuits involve a number of separate electronic or optical components on the same chip which need to be laterally defined on the chip so as to form separate components. The usual method for patterning lateral features in a semiconductor wafer is photolithography. There are many variations of the photolithographic process but, with few exceptions, a lateral feature in the finished device needs to be defined in the photoresist during the photographic step. A separate etching step then transfers the feature from the photoresist into an already grown layer.
An electronic integrated circuit and particularly an opto-electronic integrated circuit require large numbers of layers. One standard measure of circuit complexity is the number of photographic masks or mask levels required to fabricate the circuit in a repetitive sequence of photolithographic steps. Each mask level usually requires a sequence of spinning on photoresist, photographic exposure, development of the photoresist to form a mask, etching the substrate through the mask, and removing the mask. Each mask level thus causes added expense in the fabrication process and additional equipment in a production environment.
A further problem arises with photolithographic definition of opto-electronic devices, for instance lasers. Typically for single-mode optical elements including lasers, the active lasing medium is photolithographically defined to have a lateral dimension close to the optical wavelength. Perhaps thereafter cladding material is grown adjacent the defined lasing medium although sometimes air/vacuum is used as the cladding. Nonetheless, there is an interface bordering the active medium which was exposed to ambient after the etching. Such an interface is likely to have a large number of electronic surface states which degrade the operation of the laser.
One novel approach in achieving lateral patterning has been disclosed by Kapon et al in a technical article entitled "Lateral patterning of semiconductor superlattice heterostructures by epitaxial growth on nonplanar substrates" appearing in Proceedings of SPIE, volume 944, 1988 at pages 80-91. This technique relies on the different growth rates on different crystal planes. They pattern a groove in a crystal substrate and thereafter grow multiple layers both in the groove and over the surrounding planar area. Because of the differential growth rates, layer thicknesses change in the vicinity of the groove. For very thin layers, quantum well effects cause the band gaps to be dependent on layer thicknesses. Thereby, lateral bandgap variations appear over the groove cross-section, which can be used for lateral light confinement. However, this effect requires very small dimensions associated with quantum wells. Furthermore, since the lateral current confinement is very poor, it requires extra steps, such as proton bombardment, to create the necessary confinement. The alignment becomes critical in such processing.
Jaeckel et al have recently disclosed in a technical article entitled "High-power fundamental mode AlGaAs quantum well channeled substrate laser grown by molecular beam epitaxy" appearing in Applied Physics Letters, volume 55, 1989 at pages 1059-1061 a method of growing a laser in a groove. A flat-bottomed groove is formed in a (100) substrate of GaAs and thereafter a graded-index separate confinement heterostructure laser is grown primarily of AlGaAs but with a GaAs quantum well active layer. The lower AlGaAs cladding layer is uniformly doped p-type. However, the upper AlGaAs cladding layer is doped with Si, which is amphoteric so as to produce n-type doping on the {100} plane exposed outside the groove and at the bottom of the groove and produce p-type doping on the {311} A planes exposed on the side of the grooves. Thereby, there is lateral current confinement in the upper cladding layer. This technique is of limited use however. It requires a unique combination of a dopant and substrate orientation. Furthermore, the p-type doping in the upper cladding layer extends only to the sides of the groove. Thereafter, this layer is again n-type. Therefore, the upper current electrode must be defined to within the width of the groove.